double-decker vs passenger vehicle

double-decker and passenger vehicle both are nouns.

Word	Noun	Adjective	Verb	Adverb
double-decker	Yes	No	No	No
passenger vehicle	Yes	No	No	No

As nouns, double-decker and passenger vehicle are synonyms defined as:

double-decker and passenger vehicle: a vehicle carrying many passengers; used for public transport

Other synonyms of double-decker include autobus, bus, charabanc, coach, jitney, motorbus, motorcoach, omnibus.

double-decker (noun)	passenger vehicle (noun)
a vehicle carrying many passengers; used for public transport	a vehicle carrying many passengers; used for public transport